Color is one of the specifications for final liquid hydrocarbon products, such as diesel fuels, wherein lighter color is required to meet specifications. In particular, desired color level of diesel fuel after processing is less than about 2.5 as measured according ASTM D6045. Even so, oil refineries typically target a manufacturing specification at or below a color specification of 2.0. Without being bound by theory, it is believed that discoloration of diesel fuel with less than about 10 ppm of sulfur following hydrotreating may originate from aromatic compounds, in particular multi-ring aromatic compounds. For example, toward the end of a hydrotreating catalyst cycle, decreasing hydrogenation effectiveness and increasing run temperatures of the catalyst may lead to an increased presence of multi-ring aromatic compounds thereby rendering the diesel fuel off-specification (off-spec) for color.
Methods to prevent such discoloration can include decreasing hydrotreating run times and/or increasing catalyst regeneration frequency, but such methods result in hydrotreating processes with lower efficiency and are not desirable. Thus, new methods for purifying diesel fuel and improving color of diesel fuel are needed, especially diesel fuel following hydrotreating.